Tidal Swamp Forests and Woodlands
This group includes all forests and woodlands that occur in areas subject to tidal flooding along the upper tidal reaches of inner Coastal Plain rivers and their tributaries, as well as in the upper reaches of wind-tidal estuaries in southeastern Virginia. Development and persistence of forest vegetation in estuarine systems appear to be limited downstream by salinity and upstream by sufficient availability of sediment.
Tidal hardwood swamps occur along all of the major eastern Virginia rivers from the James River northward, but are most extensively developed along the Pamunkey and Mattaponi Rivers, where regular tidal inundation is unimpeded by levees or channel alteration. Its habitats are influenced by lunar tides up to 1 m (3 ft), but diluting freshwater flows from upstream keep salinity levels below 0.5 ppt. Communities in this group are structurally complex, with semi-open overstories and diverse multiple lower strata. Pumpkin ash (Fraxinus profunda) and swamp tupelo (Nyssa biflora) are the most abundant overstory species, with occasional associates of red maple (Acer rubrum), green ash (Fraxinus pennsylvanica), sweetgum (Liquidambar styraciflua), swamp chestnut oak (Quercus michauxii), common persimmon (Diospyros virginiana), and black gum (Nyssa sylvatica). From the Potomac River north, green ash becomes increasingly important and largely replaces pumpkin ash as an overstory dominant in northern Virginia. Shrub layers are mixed and extraordinarily diverse. Common species include winterberry (Ilex verticillata), smooth alder (Alnus serrulata), southern wild raisin (Viburnum nudum), arrow-wood (Viburnum dentatum), wax myrtle (Morella cerifera), American holly (Ilex opaca var. opaca), fetterbush (Eubotrys racemosus), spicebush (Lindera benzoin var. benzoin and var. pubescens), sweetbay magnolia (Magnolia virginiana var. virginiana), swamp rose (Rosa palustris), silky dogwood (Cornus amomum) stiff dogwood (Cornus foemina) , and Virginia-willow (Itea virginica). Climbing vines such as poison ivy (Toxicodendron radicans var. radicans), Virginia-creeper (Parthenocissus quinquefolia), and greenbriers (Smilax spp.) are also common. Herb layers are rich with a wide variety of wetland ferns, graminoids, and forbs. Characteristic herbs are halberd-leaf tearthumb (Persicaria arifolia), groundnut (Apios americana), rice cutgrass (Leersia oryzoides), arrow-arum (Peltandra virginica), false nettle (Boehmeria cylindrica), water-hemlock (Cicuta maculata var. maculata), Virginia dayflower (Commelina virginica), lizard's-tail (Saururus cernuus), fringed sedge (Carex crinita), orange jewelweed (Impatiens capensis), Canada mint (Mentha canadensis ), royal fern (Osmunda spectabilis), cowbane (Oxypolis rigidior), and greater marsh St. John's-wort (Triadenum walteri). Stands transitional between more closed forest and open marsh may contain abundant wild rice (Zizania aquatica var. aquatica).
An influential feature of tidal hardwood swamp habitats is a pronounced hummock-and-hollow microtopography, where raised areas above the highest tide level provide stable substrates for the establishment of trees and microhabitats for more mesophytic forest herbs. These swamp forests also support cryptic animal species such as the prothonatory warbler (Protonataria citrea) and the two-toed amphiuma (Amphiuma means), as well as more noticeable species including the bald eagle (Haliaeeatus leucocephalus). Tidal hardwood swamps are considered globally uncommon to rare and are threatened by the invasive introduced forb, marsh dewflower (Murdannia keisak), by chronic sea-level rise, and by insect pathogens. Crown dieback and tree mortality have been nearly ubiquitous phenomena in these communities for decades and have been generally attributed to sea-level rise and an upstream shift in the salinity gradient. However, in the last decade, outbreaks of the introduced Emerald Ash Borer have resulted in catastrophic mortality of dominant ash trees in many stands. In some localities, tree densities are now so low that freshwater marsh vegetation is reclaiming the habitat.
Coniferous or mixed swamp forests and woodlands dominated by bald cypress (Taxodium distichum) are known only from the upper tidal reaches of rivers in Maryland, southeastern Virginia and North Carolina. Examples are documented in Virginia from the lunar-tidal Dragon Swamp / Piankatank River (Gloucester, King and Queen, and Middlesex Counties), Chickahominy River (Charles City, James City, and New Kent Counties), and James River (Isle of Wight and Surry Counties); and the wind-tidal Northwest and North Landing Rivers (City of Chesapeake and Virginia Beach). At some sites, these communities occur in ecotones between tidal marshes and non-tidal backswamps or uplands.
In lunar-tidal stands, bald cypress (Taxodium distichum) dominates an open to very open overstory, with or without hardwood associates such as swamp tupelo (Nyssa biflora), water tupelo (Nyssa aquatica), and green ash (Fraxinus pennsylvanica). Stand structure and canopy cover range from closed forest to very open woodland. Shrub and herb layers are variable but generally contain a mixture of species characteristic of both marshes and swamps. Some well-developed tidal bald cypress forests appear floristically similar to palustrine Bald Cypress-Tupelo Swamps. Other stands have a nearly monospecific herb dominance by shoreline sedge (Carex hyalinolepis). In a unique, possibly fire-influenced, savanna-like stand on the Northwest River, the herbaceous dominants, in rough seasonal order, are silvery sedge (Carex canescens var. disjuncta), spikerushes (Eleocharis fallax and Eleocharis rostellata), rattlesnake-master (Eryngium aquaticum), and wild rice (Zizania aquatica var. aquatica).
A distinctive, mixed tidal swamp forest in extreme southeastern Virginia is subject to irregular wind-tidal flooding. As currently defined, this community type appears to be a globally rare endemic of the Embayed Region of southeastern Virginia and northeastern North Carolina; similar communities, however, occur occasionally further north on the irregularly flooded edges of lunar-tidal systems. In Virginia, stands occur primarily along the North Landing and Northwest Rivers (Cities of Virginia Beach and Chesapeake), estuarine tributaries of Currituck Sound. Although these systems are no longer influenced by lunar tides because of inlet closures, they are subject to wind-driven currents that produce as much as 1 m (3 ft) of variation in water levels and contribute to a salinity regime that fluctuates between completely fresh and about 5 ppt. This forest borders the wind-tidal marshes along the lower portions of the two rivers, extending well upstream of the limit of marshes in narrowing channel-side belts. It appears to represent a long-term seral stage in succession from marsh to swamp forest. Habitats have a pronounced hummock-and-hollow microtopography, with an average flooding depth 40 cm (16 in) above the hollow bottoms. Soils are coarse, fibric peats that appear indistinguishable from adjacent marsh peats. Swamp tupelo (Nyssa biflora), bald cypress (Taxodium distichum), and loblolly pine (Pinus taeda) are the dominant overstory trees in variable combinations. Spanish-moss (Tillandsia usneoides) is locally abundant, festooning the trees in some stands. Sweetbay magnolia (Magnolia virginiana var. virginiana) and red bay (Persea palustris) are scattered understory trees, while wax myrtle (Morella cerifera) dominates the shrub layer. The herb layer is diverse, containing species characteristic of both marshes and swamps, but royal fern (Osmunda spectabilis) often dominates.
The environmental dynamics, compositional variation, and state-wide distribution of Tidal Bald Cypress Forests and Woodlands are not well known and need intensive study.
Reference: Ahnert (1960), Coulling (2002), Doumlele et al . (1985), Fleming and Moorhead (1998), McCoy and Fleming (2000), Rheinhardt (1992) .
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© DCR-DNH, Gary P. Fleming.